Method and device for adjusting opening time of a fuel injector for mounting a direct injection internal combustion engine

ABSTRACT

According to this method, a) a series of sparks is emitted into an engine cylinder ( 1 ), filled with an air/fuel mixture to be ignited, at predetermined successive instants, b) one of the sparks that initiates the ignition of the mixture is identified, c) the difference separating the instant of emission of the spark from a predetermined ignition instant able to ensure that the engine supplies a predetermined mechanical power is evaluated, and d) the instant of opening of the fuel injector ( 3 ) is corrected as a function of the difference, so as to initiate subsequent ignitions of the air/fuel mixture in the cylinder ( 1 ) at the predetermined instant.

CROSS REFERENCE TO RELATED APPLICATION

This is the 35 USC 371 national stage of International ApplicationPCT/EP02/00282 filed on Jan. 14, 2002, which designated the UnitedStates of America.

FIELD OF THE INVENTION

The present invention relates to a method for setting the instant ofopening of a fuel injector in a cylinder of a direct-injection internalcombustion engine.

BACKGROUND OF THE INVENTION

In an engine of this type, particularly one using the “stratifiedcharge” mode of combustion, it is important for the fuel involved in thecomposition of the air/fuel mixture to be burned in the enginecylinders, to be introduced into each cylinder at a moment chosen sothat this mixture precisely reaches the electrodes of a spark plug forigniting the mixture at the moment when the electrodes emit a spark. Itis only by doing this that correct combustion of the mixture can beensured and, in particular, that effective protection can be had againstany possibility of a misfire.

Significant effort needs therefore to be put into setting the instant ofopening of the injector before such an engine can enter service, namelybefore a motor vehicle powered by this engine can be put in circulationfor example.

This setting is made necessary by inevitable manufacturing tolerancesaffecting various parts of the engine or associated with the latter.Furthermore, during the life of the engine, theinstant-of-opening-setting may drift, or need tweaking as a result, forexample, of the aging of the aforesaid parts.

These difficulties can be overcome by lengthening the time for which aspark is likely to initiate combustion of the mixture or by multiplyingthe ignition sparks in such a way that at least one of them is presentat the moment when the mixture to be burnt reaches the electrodes of thespark plug.

These measures unfortunately have the effect of shortening the workinglife of the spark plug, leading to costly replacements thereof.

Furthermore, as the actual instant of ignition of the mixture thenremains unknown, the engine management computer conventionally used isunable to evaluate precisely the torque delivered by this engine. Enginecontrol strategies based on knowledge of this torque (elimination oftorque surges, rapid reduction of torque when the transmission changesgear, for example) are then difficult, if not impossible, to implement.

SUMMARY OF THE INVENTION

It is precisely an object of the present invention to provide a methodfor setting the instant of opening of a fuel injector in a cylinder of adirect-injection internal combustion engine, that does not suffer fromthe drawbacks mentioned hereinabove and that, in particular, allows thelife of the engine spark plugs to be lengthened, and makes it possibleto maximize and estimate precisely the torque delivered by this engine,while at the same time allowing the engine to be adapted easily toproduce these results.

This object of the invention, together with others that will becomeapparent from reading the description that will follow, is achievedusing a method for setting the instant of opening of a fuel injector inat least one of the cylinders of a direct-injection internal combustionengine, notable in that: a) a series of sparks is emitted into saidcylinder, filled with an air/fuel mixture to be ignited, atpredetermined successive instants, b) the one of said sparks thatinitiates said ignition of said mixture is identified, c) the differenceseparating the instant of emission of said spark from a predeterminedignition instant able to ensure that said engine supplies apredetermined mechanical power is evaluated, and d) the instant ofopening of said fuel injector is corrected as a function of saiddifference, so as to initiate subsequent ignitions of the air/fuelmixture in said cylinder at said predetermined instant.

As will be seen in detail later on, by virtue of this correction it isthen possible, using a single spark, to trigger combustion of theair/fuel mixture in the cylinder in such a way that it extends over atime interval chosen so that the engine then produces a predeterminedperformance, in terms of delivered torque for example. It is thuspossible to set the instant of opening of the injector so that thedelivered torque is a maximum, for example, for a given engine speed andload.

The invention also provides a device for implementing this method,comprising a) means able to emit a series of sparks into the enginecylinder when this cylinder is full of an air/fuel mixture that is to beignited, b) means for detecting ignition of said mixture and deliveringa corresponding signal, and c) calculation means sensitive to saidsignal and able to control, on the one hand, said means for emittingsaid series of sparks so that said sparks are emitted at predeterminedrespective instants and, on the other hand, the instant of opening ofsaid fuel injector in said engine cylinder, on the basis of informationcontained in said signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will becomeapparent from reading the description which will follow and fromexamining the attached drawing in which:

FIG. 1 is a diagram of a device for implementing the method according tothe invention,

FIG. 2 is a graph illustrating the method according to the invention,and

FIG. 3 is a graph illustrating an alternative form of the methodaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference is made to FIG. 1 of the attached drawing which schematicallydepicts a cylinder 1 forming part of an internal combustion engine, morespecifically of the direct-injection type. Such a cylinder is equippedwith a spark plug 2, with a fuel injector 3 opening onto the internalwall of the cylinder 1 and with several valves (intake or exhaust) likethose referenced 4 ₁, 4 ₂, as is well known.

An engine management computer 5 is duly programmed to control theinstant of opening and the duration of opening of the injector 3 and theinstant at which a spark is emitted by the spark plug 2.

According to the present invention, the device further comprises anignition module 6, known per se, able selectively, under the control ofthe computer 5, to cause the spark plug to emit a spark or severalsuccessive sparks, each of these sparks being emitted according to apredetermined “advance angle” measured with respect to the top deadcenter (TDC) of a piston 7 moving in the cylinder 1, at the end of thecompression stroke compressing a mixture of air and fuel alreadyintroduced into this cylinder by appropriate control of the injector 3and of the valves 4 ₁, 4 ₂.

According to the present invention, the device further comprises meansfor detecting the combustion of the air/fuel mixture. It is known thatthis combustion, initiated by a spark emitted by the spark plug, causesa sharp rise in the pressure and temperature of the mixture, it thusbeing possible for this combustion to be detected using a sensor 8sensitive either to the pressure in the cylinder or to the ionization ofthe gases contained in the cylinder, under the effect of the increase inthe temperature of these gases. As an alternative, this sensor 8,preferably placed near the spark plug, could be replaced by a monitoringof the current passing through the spark plug 2, after the emission ofthe spark, this current obviously being dependent on the ionization ofthe gas present between the electrodes of this spark plug.

Reference is now made to the graph of FIG. 2 to describe the method forsetting the instant of opening of the injector 3, implemented using thedevice of FIG. 1.

This graph represents the change in torque delivered by one of thecylinders of the engine during the combustion of the air/fuel mixtureintroduced into this cylinder, this torque being representative of theengine output mechanical power. As is well known, the events that followone another to culminate in this combustion: opening of the injector 3,emission of a spark by the spark plug 2, occur at instants that areidentified by an angle of rotation θ, the angle reached by the enginecrankshaft that, as is known, rotates through 720° during a full “enginecycle”, in the case of a four-stroke internal combustion engine, forexample. More specifically, the instant at which an ignition spark isemitted is identified by the “ignition advance angle” α_(ign), equal tothe angular “distance” separating this instant from the instant at whichthe piston 7 passes through top dead center TDC, which passing-throughis identified by the corresponding angular position θ_(TDC) of thecrankshaft mechanically connected to this piston.

In general, in the field of internal combustion engine control, thedifference separating two successive events can be evaluated either interms of an “angular difference” or in terms of a “time difference”,these two expressions having to be considered as interchangeable in thisdescription.

As illustrated by the graph of FIG. 2, during the combustion stroke whenthe air/fuel mixture is burnt, the torque delivered to the crankshaft bythe piston 7 of the cylinder 1 increases, reaching a maximum C_(max)after the piston 7 passes through top dead center θ_(TDC). The meantorque then delivered during combustion is advantageously maximized byinitiating combustion at an “optimum” ignition advance angle α_(opti).This angle, dependent on the engine speed N and load Ch, is calculatedby the computer 5 which can therefore control the excitation of thespark plug 3 accordingly.

The angle α_(opti) may advantageously be corrected to achieve the bestpossible compromise between the need to maximize the torque while at thesame time minimizing the discharge of noxious exhaust gases into theatmosphere.

In order for the air/fuel mixture combustion stroke to progress in theideal way, in the way described hereinabove, it is appropriate for theregion of the mist of air/fuel mixture injected into the cylinder thatreaches the electrodes of the spark plug then to be under determinedpressure, temperature and fuel-concentration conditions able to ensurevery reliable ignition of this mixture.

In the case of a direct-injection engine, particularly a stratifiedcharge engine, a certain amount of time is needed for these conditionsto be reached. It is therefore necessary for the opening of the fuelinjector to precede the excitation of the spark plug (as the crankshaftpasses to the position θ_(a2) in FIG. 2) by a time interval thatcorresponds to a predetermined angular difference that defines the crankangle θ_(i2) at which this opening needs to be triggered.

Because of the manufacturing tolerances that affect the precision of theperformance of an engine and that in consequence cause the performanceof a particular engine to deviate from the nominal performance, it mayhappen that, at the nominal advance angle fixed for ignition, the idealignition conditions mentioned hereinabove are not all fulfilled, andthis may give rise to instability in the torque delivered by the engine,or even to misfires. This is why before an engine enters service itpasses through a setup stage (known as “calibration”).

A similar situation may be encountered throughout the life of theengine, because of drifts and wear that over time affect the operationof this engine.

According to the present invention, these disadvantages are alleviatedby implementing the method for setting the instant of opening of thefuel injector according to the present invention, this method allowingthe engine to “self-set” both before the engine enters service andthroughout its working life, as will be seen later on.

This setting consists in testing, for example periodically andautomatically, the ability of the air/fuel mixture to ignite at thepredetermined optimum instant, namely when the crank angle reaches thevalue θ_(a2) having passed through the nominal value θ_(i2) of the angleof opening of the injector 3. The angular difference (θ_(a2)−θ_(i2)) isthen adjusted to a nominal value leaving just enough time for the cloudof fuel introduced into the cylinder to reach the electrodes of thespark plug 2 at a pressure, a temperature and a fuel content in the airsurrounding the electrodes that are all likely to ensure initiation ofthe combustion the air/fuel mixture at the instant a spark appearsbetween the electrodes of the spark plug, namely when the crank anglereaches the value θ_(a2).

According to the invention, in order to perform this test, the computer5 controls the ignition module 6 in such a way that the latter emits aseries of sparks at predetermined successive instants corresponding forexample to crank angles θ_(a1), θ_(a2), θ_(a3), the angles θ_(a1) andθ_(a3) preferably flanking the emission angle θ_(a2) of the nominalspark.

By virtue of the signal emitted by the sensor 8, or of the monitoring ofthe current passing through the spark plug, as seen above, the computeris informed of the state of the air/fuel mixture after each spark hasbeen emitted, namely whether or not this mixture has ignited.

If the computer then recognizes, for example, that the ignition of themixture is due to the spark emitted at the angle θ_(a1), that is to sayto a spark emitted before the nominal spark (of angle θ_(a2)), itdeduces from this that the cloud of fuel has arrived at the spark plugelectrodes early. The computer 5 then, for the subsequent combustionstrokes, delays the injection of the fuel so as accordingly to delay theinstant at which the air/fuel mixture can ignite, to the preferrednominal instant (angle θ_(a2)). The magnitude Δt of this delay is set tothe value of the time taken by the crankshaft to move from the angleθ_(a1) to the angle θ_(a2), which value the computer 5 can easilyestablish from the engine speed N.

When, on the other hand, it is the spark emitted at the angle θ_(a3)that initiates combustion of the mixture, while the earlier sparksemitted at θ_(a1) and θ_(a2) have yielded nothing, the computer 5advances the opening of the injector 3 to the instant corresponding tothe angle θ_(i1), the magnitude of this advance being calculated asindicated above.

The invention therefore makes it possible to recenter the air/fuelmixture combustion stroke on the interval of time which, for a givenengine speed N and load Ch, allows this engine to produce maximumtorque, benefiting the efficiency of this engine.

It is now apparent that the invention provides other advantages:

it makes it possible to lengthen the life of the spark plug used, bycomparison with that of a spark plug systematically producing severalsparks in the initial part of each of the air/fuel mixture combustionstrokes. Indeed, once the instant of opening of the injector has beenset as described hereinabove, just one solitary spark is needed toignite the air/fuel mixtures that follow one another in each enginecylinder.

it allows the computer to evaluate, with precision, the torque deliveredby the engine, by virtue of the precise setting of the instant ofignition of the mixture, combustion of which produces this torque.

Furthermore, the setting method according to the invention can readilybe adapted to all the phases in the life of the engine. Indeed, it canbe introduced into the engine “calibration” procedure conventionallyperformed before this engine enters service. It can also be performedperiodically by the engine management computer throughout the life ofthis engine, to correct any drift due to parts losing their settings orbecoming worn.

The method according to the invention can also be applied individuallyto each of the engine cylinders, to the advantage of the overallefficiency of this engine.

As an alternative to the embodiment described hereinabove of the methodaccording to the invention, the computer may command the emission of anumber of sparks other than 3, namely 4, 5, 6 or more sparks, increasingthe number of sparks making it possible proportionately to increase the“robustness” of the method.

Also, the “learning” process according to the invention, during whichthe optimum value of the instant of opening of the injector is learntcan be performed by setting, initially, the “angular or time” differencethat separates two successive sparks in the series of sparks, to afairly high value to cover a broad range of variations in this instantof opening.

Other series of sparks may then be emitted, during later combustionstrokes, with an increasingly small “spacing”, making it possible tohome in on the optimum position of the instant of opening.

An equivalent result can be obtained by first of all emitting a seriesof a great amount of closely-spaced sparks and by repeating the emissiongradually eliminating those sparks that are emitted outside the actualcombustion stroke.

Furthermore, it is equally possible to home in on the instant ofinitiation of combustion as it is on the instant when this combustionends, by monitoring the signal delivered by the sensor 8, or the currentflowing through the spark plug 2, as was seen earlier. By thus preciselyevaluating the duration of the combustion, it is possible to get a moreprecise estimate of the torque delivered by the engine, which estimateis used by the torque management or control means, by combustiondiagnosis means or, more generally, by any engine management meanslikely to benefit from this information.

The present invention also makes it possible gradually to vary thetorque over time in order to eliminate torque surges as the transmissionof a motor vehicle powered by an internal combustion engine for examplechanges gear.

Indeed it is known that the torque delivered by such an engine varies,all other things being equal, with the value of the ignition advanceangle, the graph of FIG. 3 illustrating this “bell curve” variationcentered on the value α_(opti). The torque delivered therefore decreaseswhen the ignition advance angle deviates from the value α_(opti), eitherupwards or downwards. The torque delivered can thus be fixed at anypredetermined setpoint value C_(sp), lower than C_(max), by setting theignition advance angle to the corresponding value α_(sp) and bysubstituting this value α_(sp) for the value α_(opti) in the method forsetting the instant of opening of the injectors according to the presentinvention. The angle α_(sp) will preferably be chosen to be smaller thanα_(opti), to avoid problems of combustion instability (pinging, etc)encountered with ignition advance angles greater than α_(opti).

Of course, the invention is not restricted to the embodiment describedand depicted, which was given merely by way of example. Thus, althoughthe invention is described hereinabove in the context of astratified-charge direct-injection internal combustion engine, it wouldbe just as applicable to a homogeneous-charge direct-injection engine.

What is claimed is:
 1. A method for setting the instant of opening of afuel injector (3) in at least one (1) of the cylinders of adirect-injection internal combustion engine, notable in that: a) aseries of sparks is emitted into said cylinder (1), filled with anair/fuel mixture to be ignited, at predetermined successive instants, b)the one of said sparks that initiates said ignition of said mixture isidentified, c) the time difference separating the instant of emission ofsaid spark from a predetermined ignition instant able to ensure thatsaid engine supplies a predetermined mechanical power is evaluated, andd) the instant of opening of said fuel injector (3) is corrected as afunction of said difference, so as to initiate subsequent ignitions ofthe air/fuel mixture in said cylinder (1) at said predetermined instant.2. The method as claimed in claim 1, characterized in that saidpredetermined ignition instant ensures that said engine produces apredetermined torque.
 3. The method as claimed in claim 2, characterizedin that said predetermined torque is the maximum torque that the enginecan deliver, for a given engine speed (N) and load (Ch).
 4. The methodas claimed in claim 1, characterized in that, during successive phasesof combustion of air/fuel mixtures in said cylinder, the number and/orseparation of the sparks in said series of sparks is varied so asgradually to pinpoint the optimum instant for the opening of theinjector.
 5. The method as claimed in claim 1, characterized in that thespark in the series that signals the end of combustion is alsoidentified so as to evaluate the total combustion time and from thatdeduce an estimate of the torque delivered by the engine.
 6. A devicefor implementing the method as claimed in claim 1, characterized in thatit comprises: a) means (2, 6) able to emit a series of sparks into saidengine cylinder (1) when this cylinder is full of an air/fuel mixturethat is to be ignited, b) means (8) for detecting ignition of saidmixture and delivering a corresponding signal, and c) calculation means(5) sensitive to said signal and able to control, on the one hand, saidmeans for emitting said series of sparks so that said sparks are emittedat predetermined respective instants and, on the other hand, the instantof opening of said fuel injector (3) in said engine cylinder (1), on thebasis of information contained in said signal.
 7. The device as claimedin claim 6, characterized in that said detection means (8) consist of amember from the group formed of: a pressure sensor, an ionizationdetector, a member measuring a monitored current in the spark plug (2)of said cylinder (1).
 8. The device as claimed in claim 6, characterizedin that it forms part of the direct-injection internal combustion enginemanagement means.